You know how when you put water into a glass and tap it with a wooden spoon you can hear a music note? Well how come the one with the most water has the lowest note?

The quick answer is Resonant Frequency: http://en.wikipedia.org/wiki/Resonant_Frequency

More specifically, Acoustic: http://en.wikipedia.org/wiki/Acoustic_resonance

In short—the mass, volume and shape of a thing determines its Resonant Frequency. That’s the frequency it’ll ring at.

Interestingly you don’t need to actually knock something to get it ringing at a resonant frequency (the knocking tends to cause a “wide band” noise which is only perpetuated in the resonant frequencies). I was once disassembling an upright piano with a circular saw—don’t ask—when I turned off the saw the harp was ringing with the same frequencies as the saw + some octave tones.

Here is a classic example of resonant frequency in action.

When a glass, such as a wine glass, has more water in it, it causes the glass to vibrate at a slower frequency, causing a lower tone. When there is less water, the glass is free to vibrate at a faster rate, causing a higher tone.

Here’s an interesting related phenomenon: take an aluminum pot, and fill it with about an inch or two of water. then strike it so it clangs. Immediately tilt it as if to pour the water out. As the water rises on the side, the tone of the “clang” will go lower.

@grumpyfish Interesting story about the piano. Those “octave” tones you mentioned are called harmonics. This can be done with any string instrument. I use harmonics to tune my 12-string. If you stop the string at exactly halfway between the nut and the bridge without fretting it (in other words, without holding the string on the fingerboard), and pluck it just right, the string will sound a tone exactly one octave higher than the full string, unfretted. You can see Jeff Beck doing this in Jeff’s Boogie.